Coherent population trapping

In atomic physics, a dark state refers to a state of an atom or molecule that can not absorb (or emit) photons. All atoms and molecules are described by quantum states; different states can have different energies and a system can make a transition from one energy level to another by emitting or absorbing one or more photons. However, not all transitions between arbitrary states are allowed. A state that cannot absorb an incident photon is called a dark state. This can occur in experiments using laser light to induce transitions between energy levels, when atoms can spontaneously decay into a state that is not coupled to any other level by the laser light, preventing the atom from absorbing or emitting light from that state. A dark state can also be the result of quantum interference in a three-level system, when an atom is in a coherent superposition of two states, both of which are coupled by lasers at the right frequency to a third state. With the system in a particular superposition of the two states, the system can be made dark to both lasers as the probability of absorbing a photon goes to 0.

Experiments in atomic physics are often done with a laser of a specific frequency ${\displaystyle \omega }$ (meaning the photons have a specific energy), so they only couple one set of states with a particular energy ${\displaystyle E_{1}}$ to another set of states with an energy ${\displaystyle E_{2}=E_{1}+\hbar \omega }$. However, the atom can still decay spontaneously into a third state by emitting a photon of a different frequency. The new state with energy ${\displaystyle E_{3}<E_{2}}$ of the atom no longer interacts with the laser simply because no photons of the right frequency are present to induce a transition to a different level. In practice, the term dark state is often used for a state that is not accessible by the specific laser in use even though transitions from this state are in principle allowed.

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